The present invention relates to a process for cleaving alkyl tert.-alkyl ethers into the underlying alkanols and tert.-olefins in a column apparatus.
It is already known (DE-OS (German Published Specification) 3,210,435) to cleave methyl tert.-butyl ether (MTBE) in a column apparatus into methanol and i-butene, a cation exchanger based on sulphonated styrene/divinylbenzene resins being arranged as a catalyst in various column sections, between each of which column sections with 6 distillation trays but without cation exchanger are located. The MTBE is fed above the top catalyst bed. The flow of the liquid MTBE fed in and of the methanol produced during the reaction is downwards through the catalyst bed, and the gaseous i-butene formed, if appropriate as an azeotrope with methanol, flows upwards through the catalyst bed. With respect to the reaction conditions, the above DE-OS (German Published Specification) gives an indication of 5 bar and a column temperature between 45.degree. C. (top temperature) and 100.degree. C. (bottom temperature). In such a process, it is to be expected that 1. the column will easily flood due to the dense catalyst packing and hence severely lose separation effect and 2. the catalyst bed is continuously turned over by the liquid and gaseous streams flowing through in opposite directions, channel formation in the catalyst beds being highly probable. As a result of such channel formation, the major part of the catalyst mass would not come into contact with the substances flowing through, and its utilization would thus be completely unsatisfactory. The assembly of such a column apparatus, the introduction of the catalyst into the envisaged column sections and, of course, any replacement of the catalyst are involved and hence expensive. i-Butene, which is first formed in the lower catalyst layers, must flow on its upward path through all the catalyst layers and beds located above; during this, undesired dimerization to give diisobutylene cannot be excluded, which reduces the i-butene yield and causes additional difficulties in working up the bottom product. Similarly, methanol produced by cleavage in the upper catalyst layers is subject to the risk or formation of dimethyl ether on the acidic catalyst resin on its path to the column bottom. The formation of undesired by-products is further promoted by the high temperature in the lower part of the column.